In repairing fractured bones, it is frequently necessary to reattach bone fragments using fasteners or a series of fasteners in conjunction with a plate. Many of the fasteners used in this application require a hole to be drilled before they can be installed. While this significantly reduces the torque the fastener experiences during insertion, clinical tests have shown that up to 1/3 of the fasteners inserted with this technique establish inadequate bone/screw contact to achieve adequate connection. Other fasteners are awkward, particularly in tight places associated with operating procedures, since they require two hands to install: one to hold the fastener and the other the driver. Further, a number of supposedly self-drilling, self-tapping fasteners lack adequate strength to sustain the torque required in this application.
The self-drilling, self-tapping screw of the present invention overcomes the defects of the prior art fasteners. Its self-drilling and self-tapping features allow installation without the need to pre-drill an aperture to receive it. The process of self-drilling and self-tapping provides increased surface contact between bone and screw thread improving the holding power of the screw. In addition, the screw of the present invention removes a minimal amount of bone material prior to thread engagement. The screw is made of a material which is capable of receiving high torque, preferably a material selected from the group consisting of titanium alloys, cobalt chrome and high strength resinous polymers.
The screw has three basic sections: a tip region, a body region and a head region. The tip region has a point with an included angle of between 45.degree. and 50.degree. and more preferably, between 46.degree. and 49.degree., a compromise between penetrability and strength requirements. The tip region has a retention thread that extends substantially throughout its entire length. The body region has a substantially uniform diameter and a retention thread which also extends substantially throughout the entire length of the body region. The head region includes a head which has a drive recess and a portion underlying said head which is threadless.
The head region has been specifically designed to maximize drive torque to enable the screw to penetrate the hard cortical bone portion while minimizing the material removed from the head so as not to unduly weaken the integrity of the head. Removal of excess material can result in a weakened head region which can result in the head twisting off the body or the driver stripping the recess. The drive recess is cruciform in shape and extends from between 70 and 95% of the maximum diameter of the head to increase surface contact with the driver and, thereby, the torque transfer capability. Since the recess does not extend the full width, the outer periphery of the head maintains its integrity as a cylinder of metal which has significantly greater strength and resistance to stripping than it otherwise would have. The bottom of the recess is arcuate to minimize the amount of material removed from the head in forming the recess. The recess has retention means so said screw can be attached to a driver and inserted by a single hand of a user. The retention means preferably takes the form of an interference fit with the driver. A cutting flute extends from the tip region into the body region and removes bone fragments to permit penetration and thread formation by the screw in at least one of the bone sections which are being reattached by said screw.
A preferred but optional feature of the screw is that the head have an included angle .delta. of at least about 90.degree. to prevent pull through of the screw while minimizing its profile. Additionally, it is a preferred feature the threads on both the tip region and body region have substantially similar shape and pitch, including that the lead angle of each be between 48.degree. and 52.degree. and the trailing angle be between 98.degree. and 102.degree. as measured from the centerline of the screw. More preferably, the retention thread of said tip region has a thread pitch which is slightly smaller than the thread pitch of said retention thread of said body region.
Various other features, advantages and characteristics of the present invention will become apparent to one of ordinary skill in the art after a reading of the following specification.